You need some tools to help you.
This is a tire calculator to help you sort out how much your speedometer will be affected.
This is Discount Tire's web site, and not only can they tell you what your vehicle originally took for tire size (which goes to the speedometer error), but there is also a place to look up alternative tire sizes that will fit.
Word of warning: The alternative tire size fitment doesn't take into account the change in rim width that might be needed to accommodate the tire size listyed - so you need to check that to be sure you get that right.
How do you find your current rim width? It is stamped on the back of the wheel somewhere. It will take the form "16X6JJ" - although the letters might not be there. The 16" is the rim diameter, the 6 is the rim width, and the JJ is a code for the type of rim flange.
But if your vehicle has stock wheels, there are lists for that info. It is unfortunate that you didn't tell us what your vehicle is (year, make, and model) because I have one of those lists.
And lastly, lowering the car is obviously going change how much space is available within the fenderwell, so the Discount Tire Web site will be wrong - but it is at least a good guide as to what DOESN'T fit.